Content hosting services host and deliver digital content items, such as videos, to users. Users provide content to the hosting service, and that content may include portions that are user generated as well as portions that are not generated by the user. In the latter case, a content owner other than the user may have rights in the content, thus making it desirable to detect the presence of that content in user-uploaded videos. To help detect such content items, rights holders often provide reference content to the content hosting service. The content hosting service then attempts to match content received from users with reference content, and when a match occurs, the content hosting site can take an appropriate action. In some cases, such reference content items themselves may inadvertently include third party content to which the entity that provided the reference content item does not have rights, thus leading to incorrect matching and flagging of the user generated content.